The long-term objective of this proposal is to identify interactions between xenobiotics and CYP26 enzymes that lead to altered all-trans retinoic acid (RA) concentrations in plasma and in different tissues. This is important because accurate control of RA concentrations is critical for reproduction, fetal and placental development, maintenance of epithelia, regulation of immunity and apoptosis and cancer prevention and treatment. Compounds that alter RA elimination by inhibiting or inducing the enzymes responsible for RA metabolism have the potential to cause multiple detrimental effects on individuals' health. The central hypothesis of this grant proposal is that CYP26 enzymes regulate circulating RA concentrations and control the clearance of RA in vivo. This hypothesis will be tested by the three specific aims designed to provide a comprehensive characterization of the role of CYP26A1 and CYP26B1 in regulating RA homeostasis. The first specific aim of this proposal is to demonstrate that CYP26A1 and CYP26B1 are the major RA hydroxylases in adult human tissues. The second specific aim is to identify xenobiotic and endogenous ligands of CYP26A1 and CYP26B1. Aim 3 will test the effect of selected xenobiotics on CYP26A1 and CYP26B1 expression and RA metabolism in human tissues and cell lines. The results of these aims will provide important basic understanding of the function and substrate specificity of CYP26A1 and CYP26B1, two understudied and poorly characterized P450 enzymes. Tight regulation of cellular retinoic acid concentrations is essential for normal reproduction, immune competence, maintenance of healthy epithelia and bone health and for regulation of apoptosis and cell division. As such, alteration of the processes that control cellular retinoic acid metabolism can cause multiple detrimental effects on an individual's health. This proposal aims at characterizing interactions between xenobiotics and retinoic acid metabolizing CYP26 enzymes that alter retinoic acid metabolism and clearance and can lead to adverse effects.